Per Oci-One Kanubi:
It is
human nature to avoid going to a great effort or putting oneself to
great discomfort, and stripping to the waist to take a leak is a
measurable effort and quite uncomfortable in cold weather (uhhhh,

I find it the opposite. If I'm dressed for any kind of
prolonged immersion, I'm sweating pretty heavily while paddling
and it's a relief to pull out, and strip down the top for awhile.

In fact, I usually take off my shirts and hang them up to dry a
little while I'm doing what I have to do.

I still think you have a valid point about subconsciously
avoiding hydration though - but I wouldn't use my relief zipper
out on the water.... too fearful of something happening that
would flood the suit.
--
PeteCresswell